This invention relates to a measurement circuit for, and a method of, measuring at least one of the rotational position and rotational speed of the rotor of an electric motor, and to related motor control circuits and electric power assisted steering systems.
The field of control of electric motors, such as may be used in such fields as driving electric power assisted steering (EPAS) systems, has been well explored in the prior art.
In motor control, in order to correctly control the operation of the motor, it is often required to be able to measure the position of the motor rotor, for example in order to switch the current supply to different phases of a multiple-phase motor. One motor commonly used in EPAS systems is the permanent magnet synchronous motor. This comprises a plurality of permanent magnets mounted on the motor rotor, with a plurality of phase windings surrounding the rotor to form the stator. By cyclically energising the phase windings, the rotor can be forced to rotate.
Such motors often have relatively low resolution position sensors, such as Hall-effect sensors mounted relative to the stator, which can indicate the passing of a permanent magnet pole on the rotor, and so indicate to a motor control circuit the appropriate moment to switch (or “commutate”) the energisation of the phase windings.
However, it has been desired to remove the need to provide such sensors, and potentially to measure the rotational speed of the rotor also. One method of doing so is to determine the inductance of the motor by measuring the rate of change of motor current, as proposed in the PCT patent application published as WO2004/023639. It is desirable to improve the performance of such position sensing techniques.